The treatment of soils contaminated with halogenated organic compounds such as polychlorinated biphenyl compounds (hereinafter PCB's) with solvated electron solutions for soil decontamination purposes is disclosed in U.S. Pat. Nos. 4,853,040 and 5,110,364 issued in the names of Mazur et al. Although quite effective as a method for decontaminating PCB-contaminated soils, the prior art processes sometimes can be economically disadvantageous because of the problems associated with the separation of solvent from treated soil. In the case of those prior art processes, which processes use anhydrous liquid ammonia as a solvent, the economic disadvantages arise out of the use of solvent evaporation as the preferred method of separating ammonia solvent from the treated solids.
Vaporization of solvent as a means of effecting solvent separation from a solvent-treated soil is undesirable for several reasons. First, vaporization is a relatively slow process and results in slow process batch turn-around time. Second, vaporization requires substantial amounts of process heat input, thereby resulting in an unacceptably large energy economic penalty for solvent recovery. Third, vaporization recovery requires the use of expensive compression, condensation, and chilling equipment to thereby avoid the necessity of either venting or destroying the process ammonia. Also, vaporization of liquid ammonia can cause the process temperature of both the liquid ammonia and treated soil to drop because of the accompanying ammonia refrigeration effect. Such, if substantial, can result in freezing of the soil, ice formation on the process reactor vessel, and unacceptable thermal stresses in the entire soil treatment system.
Separation of the ammonia solvent, while in the liquid phase, from the treated soil, presents a unique set of problems that differs from the set of problems associated with solvent separation by evaporation. Known liquid separation approaches include the use of such mechanisms as filter belt presses, centrifuges, decanting systems, and conventional filters. Each such approach, however, is unsatisfactory in comparison to the present invention for one or more reasons.
For instance, filter belt presses are normally designed for ambient (atmospheric) pressure operation and typically leave from five to fifteen weight percent of solvent liquid in the treated soil. Although equipment design modifications for pressurized operation might be effected, such would greatly increase the complexity of, and cost associated with, liquid solvent separation.
Centrifuges, on the other hand, are able to achieve significantly higher levels of liquid removal than filter belt presses, but are disadvantageous in that they require pressurized rotary seals, substantial drive motors, higher capital equipment investment, and increased maintenance and repair costs.
In the case of decanting systems, only free-standing liquid is typically removed and thus essentially dry cake recovery is unobtainable. Lastly, conventional filters are not designed to separate liquids from slurries containing substantial quantities of a non-homogenous particulate material. Representative soils to which pressurized decontamination treatment and solvent separation using the present invention pertains, have particle sizes in the range from below 0.002 millimeter average diameter to as much as 2 millimeter average diameter with a requirement that the filtration process screen out those soil particles which are at the low size end of the soil particle size spectrum. Because a large percentage of the material to be separated is typically substantially larger than the smallest particle size, filter clogging is a major potential problem and effectively precludes the use of conventional filters for the soil treatment and liquid solvent separation application.
My invention avoids the above-discussed economic constraints associated with the conventional technical approaches to solvent removal from treated soil/solvent mixture slurries, and does so in a clearly cost-effective manner.